mineral

Optical Calcite

Calcite (Calcium Carbonate, CaCO3)

Hardness: 3 (Mohs scale); Color: Colorless/transparent; Luster: Vitreous to pearly; Crystal Structure: Trigonal/rhombohedral; Cleavage: Perfect rhombohedral in three directions; Specific Gravity: 2.71

Hardness: 3 (Mohs scale)
Color: Colorless/transparent
Luster: Vitreous to pearly

Identified July 8, 2026More mineral →

Explore Optical Calcite in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Forms through chemical precipitation in sedimentary environments, hydrothermal veins, or via biological processes in marine life. It is the primary component of limestone and marble.

Uses & applications

Used in high-quality optical instruments (polarizing prisms), manufacturing cement, agricultural soil treatment, and as a popular specimen for collectors due to its unique optical properties.

Geological facts

Also known as Iceland Spar, this variety of calcite is famous for exhibiting strong double refraction (birefringence), where light passing through it splits into two rays, making text appear double when viewed through the crystal.

Field identification & locations

Identify by its perfect rhombohedral cleavage (slanted cube shape), its soft surface that can be scratched by a copper coin, and its reaction (fizzing) to weak acids like vinegar. Common in many global locations including Iceland, Mexico, and the USA.